In recent years, thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye donor element is placed face-to-face with a dye receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen.
Thermal prints are susceptible to retransfer of dyes to adjacent surfaces and to discoloration by fingerprints. This is due to dye being at the surface of the dye receiving layer of the print. These dyes can be driven further into the dye receiving layer by thermally fusing the print with either hot rollers or a thermal head. This will help to reduce dye retransfer and fingerprint susceptibility, but does not eliminate these problems. The application of a protective overcoat, however, will practically eliminate these concerns.
U.S. Pat. No. 4,522,881 discloses a protective cover film for thermal dye transfer prints comprising a UV protective layer but gives no guidance for the selection of binder resins to optimize print gloss.
U.S. Pat. No. 5,332,713 discloses a dye donor element for thermal dye transfer, and more particularly to the use of a transferable protection overcoat in the element for transfer to a thermal print. The dye donor element includes a support with at least one dye layer area comprising an image dye in a binder and another area comprising a transferable protection layer of approximately equal size. The transferable protection layer comprises poly(vinyl formal), poly(vinyl benzal) or poly(vinyl acetal) containing at least about 5 mole % hydroxyl. There is a problem with these materials, however, in that they provide inferior gloss and iridescence performance due to refractive index mismatch with the dye receiving layer.
U.S. Pat. No. 5,387,573 relates to a dye donor element for thermal dye transfer, and more particularly to the use of particles in a transferable protection overcoat in the element for transfer to a thermal print. The dye donor element includes a support having bearing at least one dye layer area comprising an image dye in a binder and another area comprising a transferable protection layer of approximately equal size. The transferable protection layer contains particles in an amount of up to about 75% of the thickness of the transferable protection layer. Although the particles eliminate the iridescence problems of the prior art, the particles lower the gloss of the imaged print.
U.S. Pat. No. 5,514,637 relates to a dye donor element for thermal dye transfer, and more particularly to the use of a transferable polymeric protection overcoat having a certain molecular weight in the element for transfer to a thermal print. The dye donor element includes a support bearing at least one dye layer area comprising an image dye in a binder and another area comprising a transferable protection layer of approximately equal size. The transferable protection layer comprises poly(vinyl benzal) having a polymethylmethacrylate equivalent molecular weight of less than about 75,000 as measured by size exclusion chromatography or poly(vinyl acetal) having a polymethylmethacrylate equivalent molecular weight of less than about 65,000 as measured by size exclusion chromatography. Although this overcoat provides a clean break-off performance between transferred and non-transferred laminate, gloss performance is poor.
U.S. Pat. No. 5,668,081 discloses a protective sheet comprising poly(vinyl formal), poly(vinyl benzal) or poly(vinyl acetal) containing at least about 5 mole % hydroxyl, and a cross-linking agent, or a phenoxy resin and a cross-linking agent. These laminates provide improved chemical degradation sensitivity, but they are difficult to manufacture. These non-benzoated phenoxy resins show poor solubility in the solvents desirable for cost-saving, such as methanol and toluene.
U.S. Pat. No. 5,670,449 discloses the use of elastomeric beads in a protective overcoat for better raw-stock keeping, but the gloss performance of these laminates is not optimum.
U.S. Pat. No. 6,942,956 discloses a protective laminate comprising a gloss-enhancing agent, and a mixture of inorganic and organic particles. In a preferred embodiment of the invention, the protection layer contains from about 5% to about 60% by weight inorganic particles, from about 25% to about 80% by weight polymeric binder and from about 5% to about 60% by weight of organic particles, and an effective amount of at least one gloss-enhancing compound. The gloss enhancing compound consists of an organic molecule that is essentially colorless, does not scatter light, is substantially not absorbing of light at a wavelength from 400 to 800 nm, and has a maximum absorption at a wavelength less than 400 nm. The inorganic particles, e.g. silica, are required to provide smooth laminate tear-off, but these degrade gloss and are detrimental to the gravure coating quality. The organic particles are required to reduce iridescence, but these reduce gloss. The gloss improvement provided is not adequate.
U.S. Pat. Application No. 2006/0068174 discloses a protective sheet comprising a peeling layer and an adhesive layer, with specific refractive index requirements between the two layers to prevent iridescence. This two-layer laminate adds to manufacturing cost, and is not optimum for both gloss and image stability.
U.S. Pat. No. 6,733,611 discloses the use of a transferable protection layer containing a resin having a Tg value of 40-100° C. and a storage modulus at 110° C. of not more than 1×105 Pa, but makes no mention of cut edge quality or material solubility.
Japanese Pat. No. 2,825,778 discloses a protective laminate comprising an ultraviolet absorber with methyl methacrylate, styrene acrylate, a cellulosic resin or a mixture thereof having a glass transition point of 100° C. These acrylate laminates have poor physical properties, and styrene-containing laminate give poor barrier properties for pollutant gases.
Japanese Kokai No. 2006/021402 discloses a protective sheet comprising an acrylate copolymer resin, but these laminates have inadequate physical properties.
Japanese Kokai No. 2006/095982 discloses a protective sheet to prevent iridescence with a transferable layer of a material with a specific refractive index range which depends on the film thickness, but the resins disclosed are not optimum for both gloss and image stability.